O-ring package

ABSTRACT

A transfer molded plastic package having a cavity for accommodating a semiconductor chip is disclosed. A leadframe assembly process is shown wherein the leadframe finger pattern is provided with a resilient or elastic O-ring bead. Top and bottom housing plates which have dimensions that are larger than the bead form the upper and lower surfaces of the package. These plates can be formed of any suitably rigid material. They may be composed of ceramic in low power devices. For high power operation at least one metal plate can be employed. The chip or chips are connected to the lead frame and, along with the top and bottom plates, is located in a transfer mold. The plates are in registy and located so that their outer edges extend beyond the O-ring bead. The mold cavities include faces which press against the plates which are held apart by the O-ring bead so that the bead is compressed by the mold closure. A plastic encapsulant is molded around the periphery of the plates so that the leadframe fingers are secured therein to become package pins.

BACKGROUND OF THE INVENTION

This is a continuation-in-part of application Ser. No. 610,640, filedNov. 8, 1990, (now abandoned).

The invention relates generally to the field of semiconductor devicepackaging and specifically relates to the transfer molding of plasticencapsulated devices. Ceramic hermetic semiconductor packaging is thepreferred way of high reliability packaging because it offers severaldesired advantages Mainly, once sealed, the package is substantiallyimpervious to its environment. The final seal can be accomplished in anenvironment that is not stressful to the semiconductor device and thisenvironment will be maintained throughout the life of the device.However, such packages have proven to be expensive. In many cases thecost of the package greatly exceeds the cost of producing the chip thatit houses. On the other hand, the well known plastic moldedsemiconductor devices can be produced cheaply. Typically, the chip beinghoused costs more to produce than the package and this is a desirableeconomic condition. Plastic encapsulation, while cheap and easy toaccomplish, has several disadvantages. For most of these is the problemthat such encapsulation is not hermetic. Such packaging allows the longterm entry of environmental elements which can adversely affect asemiconductor chip. While this is a difficult problem, the producers ofchips have advanced their use of protective seal coatings to a pointwhere hermetic sealing is not necessary. The reliability of plasticencapsulated semiconductor chips in the presence of adverse environmentshas advanced to a level where hermetic packaging is not alwaysattractive. Certainly the cost/benefit relationship now militatesagainst hermetic packaging. This leaves the device designer with theother problems associated with transfer molding. These include theproblems of stress which develops when the plastic encapsulant comesinto contact with the semiconductor chip face. Such stress, in theextreme, can result in chip fracture during temperature cycling. Also,some semiconductor chips are sensitive to stress and their operatingcharacteristics will change during encapsulation. Finally, manysemiconductor chips require post assembly characterization such as PAL(programmed array logic) devices. In one such heirarchy, fuses are blownto disconnect crossbar switch arrays to produce a desired pattern. Withplastic encapsulation, fuse blowing is difficult because the metalvapors thus produced have nowhere to go. Consequently, such programmingis done only on cavity-type ceramic packaged devices. It would bedesirable to have a cavity type plastic encapsulated package.

In the transfer molding of plastic encapsulated devices, it is common toemploy a dambar on the leadframe to control mold flash. The dambarstructure is designed to mate with the edge of the cavity in the moldingdie which creates the final plastic block. After the molding iscompleted the dambar segments that join the leads together are removedso that the leads are functionally separate. This requires a separatefabrication step and, with the high lead count packages now beingfavored, can be a difficult task. In fact, when the device dimensionsreduce the lead spacing to about 12 mils (0.3 mm) mechanical punchesbecome impractical. At this point, it has become standard practice touse a narrow focussed laser beam to do the cutting. In the squarepackages, now becoming popular, high lead count packages having 150 to200 pins result in spacings that make dambar removal very difficult.Accordingly, a modification in the dambar system would be desirable.

SUMMARY OF THE INVENTION

It is an object of the invention to incorporate an O-ring onto theleadframe that is to be used to connect a semiconductor device to itspins in a plastic molded structure wherein the O-ring prevents moldingcompound from contacting the semiconductor device.

It is a further object of the invention to employ a pair of plates toform a semiconductor device package and to separate the plates by meansof a compliant elastic bead or O-ring formed on the package leadframewhereby the plate spacing is maintained to clear the semiconductordevice incorporated onto the leadframe and the edges of the plates canbe joined together with a molded plastic ring.

It is a still further object of the invention to employ metal plates tocreate a semiconductor device package where the plates are spaced apartby means of an O-ring bead incorporated into the leadframe and thesemiconductor device can be associated intimately with a metal plate tocreate a high power device.

It is a still further object of the invention to employ a dambarlessleadframe for producing a plastic molded semiconductor device package inwhich an O-ring bead is located upon the leadframe to prevent mold flashand thereby to define the molded plastic outline.

These and other objects are achieved in the following manner. Asemiconductor package is created using a pair of relatively rigid platesto define the upper and lower package surfaces. These plates can be madeof any suitably rigid material. For example, they can be composed of lowcost ceramic, or they can be composed of metal where high powerdissipation is desired. The plates are spaced apart by means of aresilient or elastic insulating O-ring that is located upon theleadframe that is to be employed in creating the package. The O-ring isformed upon the leadframe outboard of the semiconductor chip region andinboard of the edges of the plates to be employed in creating thepackage. The semiconductor chip (or chips) is associated with theleadframe and the leadframe fingers connected to the chip bonding pads.Then, the leadframe and semiconductor chip are associated with one ofthe package plates and the assembly, along with an oriented secondplate, is placed in a transfer mold which has a cavity that willaccommodate the assembly. The mold cavity faces press against the plateswhich are therfore pressed against the leadframe bead so that the O-ringis compressed and therefore holds them separated on either side of thesemiconductor chip. Then a conventional plastic ring is transfer moldedaround the edges of the plates so that the lead frame fingers extendtherethrough in the conventional manner. In this molding system theO-ring bead will prevent the ingress of molding compound and the plates,along with the bead, will form a cavity around the semiconductor chip.The periphery of the plates will be surrounded by the molded plasticwhich will also join them together to create a unitary packagestructure. If desired, the plate edges can be shaped to key into theplastic so that they are rigidly retained.

When a high lead-count package is to be created a leadframe having nodambars can be employed. Here an insulating resilient or elastic O-ringis formed in the region where a dambar would ordinarily be located. ThisO-ring is made relatively thin and in registry with the encapsulationmolding die cavity edges. When the molding die is closed the cavityedges mate with the O-ring which is thereby compressed and will act topreclude molding flash. Thus, the O-ring will define the outer edges ofthe molded plastic and the conventional dambars are not needed.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is not to scale. Dimensions have been exaggerated to moreclearly portray the invention.

FIG. 1 is a front elevation view of a square package using theinvention.

FIG. 2 is a cross-section of the FIG. 1 structure take at line 2--2.

FIGS. 3A, 3B and 3C are an enlarged views of the seal region of the FIG.2 showing taken at line 3--3. FIG. 3A shows a seal made using metalplates that are punched out of sheet stock, FIG. 3B shows a seal madeusing plates having reentrant formed edges--, and FIG. 3C shows a formof o-ring construction in which the lower portion is a flat tape segmentthat is overlaid with a compliant bead.

FIG. 4 is a block diagram showing the process for practicing theinvention.

FIG. 5 is a segment of a metal leadframe showing the finger structureand the location of a compliant O-ring which operates to create thesemiconductor device package.

FIG. 6 is a fragmentary cross section of ring 15 taken at the linedesignated 6--6.

FIG. 7 is a segment of a metal leadframe that has no dambars andincludes a plastic O-ring in place of the conventional mold-flashpreventing dambar.

DESCRIPTION OF THE INVENTION

The invention is embodied in the semiconductor device package 10portrayed in FIG. 1. A plate 11, which is flush with the upper surface,is held in registry with a bottom plate (not visible in FIG. 1) by meansof a plastic ring 12 molded in place. Leads 13 pass through the plasticring in the manner well known in the plastic encapsulated package art.The leads 13 are created in the conventional leadframe manner andencased conventionally in molded plastic ring 12. While copper ispreferred as the leadframe material, other suitable metals can beemployed.

FIG. 2 is a cross-section of the FIG. 1 package 10 taken at line 2--2.FIGS. 3A, 3B and 3C show enlarged alternative forms of the plastic sealregion of FIG. 2. Top plate 11, with its bottom plate 14 counterpart,create a cavity-type of package by virtue of a resilient plastic bead 15which was formed on the leadframe that incorporates leads 13 prior toassembly.

In FIG. 3A plates 11 and 14 have sloped edges that can be created by thepunch faces employed in the metal plate punching operation. In FIG. 3Bplates 11' and 14' have reentrant edges created typically by a machiningoperation. In either form the edge shape will cooperate with plasticring 12 to key the plates into their final position.

In FIG. 3C the resilient bead 15 is applied to only the upper side ofleads 13. The bottom side of leads 13 are provide with a flat ring 15'of suitable tape such as sheet polyimide.

FIG. 4 is a block diagram of the preferred package assembly process. Thecritical step of the process is at block 19 where the O-ring is applied.A semiconductor device chip 16 is prepared conventionally and is mountedupon a leadframe bonding pad 17 which in turn is associated with bottomplate 14. The bonding pads on chip 16 are connected to the leadframefingers 13 by means of wires 18.

The package 10 is created in the following preferred manner. First, aleadframe is prepared in the usual manner and a resilient plastic bead15 applied in the form of a ring that surrounds the chip bonding pad sothat the inner extensions of the leadframe fingers extend inside thering. The ring has a diameter that is smaller than plates 11 and 14.Desirably, the ring is formed of a polyimide such as Dow Corning R-6102which is purchased as a liquid monomer. The monomer can be silk-screenedonto the leadframe or deposited by way of a moving syringe. If desired,it can be molded in place on the leadframe. After a suitable monomer hasbeen applied it is cured in position in accordance with the preferredheating schedule. For example, heating for one hour at 70° C. isfollowed by two hours at 150° C. Alternatively, an RTV silicon rubbercan be applied in liquid form and cured in place.

As shown in the alternative embodiment of FIG. 3C, the ring can beproduced by first cementing a flat plastic ring to the underside ofleads 13 and then applying the bead 15, as described above, to thetopside of the leads. Plastic ring 15' is preferred to be sheetpolyimide coated with B-staged epoxy which will glue the faces to boththe leads 13 and button plate 14. In this embodiment of the inventionthe bonding pad 17 can be eliminated, if desired, and chip 16 secureddirectly to bottom plate 14.

Ring 15 is made sufficiently thick that plates 11 and 14 must be pressedagainst it to provide the desired package thickness. This compression isachieved inside the plastic encapsulation transfer mold which is used toapply ring 12, as will be described hereinafter. After bead 15 is formedon the leadframe, semiconductor chip 16 is attached to the leadframebonding pad. (While the preferred O-ring application is shown prior tochip attachment, it can, if desired, be applied after the semiconductorchip is associated with the leadframe. This alternative is shown by thedashed line in FIG. 4.) The chip bonding pads are connected to theleadframe fingers inside the plastic ring by conventional wire bonds.Suitable clearance between wires 18 and top plate 11 is provided bymaking bead 15 sufficiently thick. However, if desired upper plate 11can be provided with a recess to accommodate the wires. Where aluminumwire bonding is to be employed, the leadframe bonding pad and the innerends of the metal fingers may be coated with a layer of aluminum. Thisprovides a suitable surface for the copper leadframe which will readilyaccept ultrasonically bonded aluminum wires.

Alternatively, thermocompression bonding can be employed to connect thechip bonding pads to the tape fingers. In this assembly method theleadframe bonding pad can be eliminated. In this operation, whichrelates to tape assembly bonding (TAB), a thin copper intermediateleadframe is created to have an inwardly extending finger pattern thatmates with the chip bonding pads. These finger patterns have anoutwardly extending form which mates with the leadframe fingers. First,the TAB fingers are thermocompression gang bonded to the chip bondingpads in an innerlead bond. Then, the chip and its associated fingers areexcised from the TAB assembly tape. The chip-associated fingers are thenthermocompression gang bonded to the leadframe fingers in an outer leadbond. In the TAB assembly the connections between the chip and itspackage are flat and very little clearance is required below top plate11. Furthermore, if desired, the leadframe fingers can be so constructedthat they mate directly with the IC chip bonding pads. Thus, if desired,the chip can be directly associated with the leadframe using TAB and theintermediate leadframe dispensed with.

It is to be understood that while a single semiconductor chip isillustrated, multichip structures can be employed. Here several chipsare attached to the bonding pad (or directly to plate 14) andinterconnected to the leadframe fingers as described. Thus, relativelycomplex circuitry can be accommodated.

After the chip is mounted and connected to the leadframe tape fingers,the leadframe bonding pad is secured to bottom plate 14. Alternatively,if the leadframe bonding pad has been eliminated, as mentioned above,the chip 16 is attached directly to lower plate 14. This is donetypically by soldering, but the use of an adhesive bond is available, ifdesired. For example, a silver particle loaded polyimide cement willprovide a suitable thermal conductivity and a reliable resilient longterm stable assembly. Such a bond is very useful in the case where thethermal expansion of bottom plate 14 is substantially different fromthat of chip 16. If desired, the O-ring 15 can be cemented to the bottomplate by one or more beads of cement thereby forming a well-establishedassembly.

In the case of FIG. 3C where the bottom portion 15' of O-ring 15 is atape, the B-staged epoxy face can be employed to bond it to plate 14.

Then the tape with the attached chip 16 and bottom plate 10 is placed onthe lower platen in an encapsulation transfer mold. The mold platen hasa recess or cavity in which the broad face is larger than lower plate14. Top plate 11 is then placed on top of the assembly in registry withbottom plate 14. If desired, the top plate can also be cemented in placeon leadframe O-ring bead 15. The transfer mold is then closed so that acavity in the upper platen is in registry with the lower platen cavity.The transfer mold platens are constructed so that when closed the cavityfaces are closer together than the sandwich comprised of the bead 15along with plates 11 and 14. Thus, the assembly is compressed so thatthe bead 15 is squeezed to flatten it. This action provides a seal thatwill preclude the entry of fluid encapsulant during transfer molding.The fit between the plates and the mold cavity faces is such that thefluid encapsulant is also excluded from the plate faces. Accordingly,when the fluid encapsulant is forced into the mold cavity it creates aring 12 of plastic encapsulant surrounding plates 11 and 14.

FIG. 5 shows a leadframe pattern suitable for practicing the invention.The fragmental drawing shows only one corner of the leadframe fingerpattern for clarity. The drawing shows a pattern segment that will berepeated seven more times to develop a single complete finger pattern.Therefore, while twelve fingers are illustrated, the finished packagewill have 96 pins arrayed on four sides as is generally illustrated inthe 20-pin package structure of FIG. 1. The leadframe chip bonding pad17 is attached at its four corners to fingers, one of which is shown at21. Four such fingers will join pad 17 to the main tape segment 20. Itis to be understood that if TAB spider bonding is to be employed, asdescribed above, the pattern shown in FIG. 5 would exclude pad 17 andfinger 21. Plastic bead 15 is applied to the tape fingers as describedabove and its location is inside dashed line 22 which represents theedges of upper plate 11 and lower plate 14. The finger pattern includesan array of inner ends that stop short of bonding pad 17 and outerextensions 13 which will ultimately form the package pins. The fingerpattern optionally includes a row 23 of enlarged sections each one ofwhich includes a central hole. Also, a series of metal links at 24 jointhe adjacent fingers together so that a dambar is created. When thepackage is transfer molded, the mold includes a cavity having edges thatare in registry with the left hand end of dambar 24. Thus, when thetransfer molding is operated, the dambar prevents a mold flash byconstraining the exit of fluid molding compound. When the transfermolding is performed the plastic encapsulant will pass through the holesin the fingers so that after curing the leadframe will be securely keyedto the plastic.

It will be noted that a second dambar type of structure is present at25. This bar is optionally employed where fingers 13 are relatively longand acts to maintain the fingers in their desired location duringhandling.

After the transfer molding is accomplished the plastic is cured so thatthe plastic ring 12 is complete with metal fingers 13 extendingoutwardly therefrom. Then the assembly is excised from the assembly tapeand the dambar elements are removed by such means as punching, clippingor laser beam vaporization. This latter method is the one of choice forthe highest lead-count packages. In fact, the ability to remove thedambars is one of the main limits in producing the higher lead-countstructures. After excision, the package leads are formed to a desiredshape, cleaned and provided with a corrosion resistant coating. Thepackage is then ready for use.

FIG. 7 shows a tape structure in which the dambars are eliminated. Thisis known as the dambarless tape assembly method and structure. Thedrawing is similar to that of FIG. 5 except for the lack of dambars. Itis to be understood that while the dambarless tape assembly is ofgreatest use on large lead count packaging where dambar removal presentsproblems, it can be employed in any transfer molded process wheremolding flash is to be avoided.

As shown in FIG. 7, a plastic bead 27 is applied to the leadframe whichis shown as being generally similar to that of FIG. 5. Siderail 26 joinsthe outer ends of fingers 13 and bead 27 holds the inner ends of thefingers in position. If the inner extremities are to be further securedin position, a polyimide tape (not shown) could be applied in theconventional way at location 22 to ensure finger stability. As usual, asemiconductor chip is mounted on lead frame bonding pad 17 inside thearea bounded by dashed line 16'. Then the chip bonding pads areconnected to the inner ends of the tape fingers by means of wires (or aTAB spider could be employed) as described above for FIG. 5. Thetransfer mold that is to apply encapsulation compound to form thepackage housing, includes recesses that define the package and theserecesses have edges that mate with the inner edges of bead 27. When thetransfer mold closes on the tape, with its bead and mountedsemiconductor chip, the resilient bead will deform and seal the moldcavity. When the fluid encapsulant is forced into the mold the action ofbead 27 will be to preclude the escape of any encapsulant outside of themold. Thus, molding flash is avoided without resorting to dambars.Finally, if desired, after encapsulation, bead 27 can be removed.However, since it is an insulator, it can be left in place in thefinished product.

It is to be understood that the dambarless approach can be employed byitself on any plastic encapsulated product. It can also be employed withthe assembly structure and process disclosed in conjunction with FIGS. 1through 5. In this operation two beads would be applied--one at location27, as shown, and a second bead would be applied inboard of location 22as is set forth in FIG. 5 as element 15. It is clear that these twobeads would fully secure the tape fingers in place even though there areno dambars. Then the assembly process associated above, with respect toFIGS. 1 through 5, would be followed except that no subsequent dambarremoval would be required. The molded plastic cavity package of FIG. 1would result. The only difference would be the inclusion of a plasticbead on fingers 13 surrounding the plastic ring 12.

The invention has been described and a preferred embodiment detailed.Alternatives have also been described. When a person skilled in the artreads the foregoing description, other alternatives and equivalents,within the spirit and intent of the invention, will be apparent.Accordingly, it is intended that the scope of the invention be limitedonly by the claims that follow.

I claim:
 1. A semiconductor chip package comprising:a lead frame havingfinger portion extensions that terminate inwardly in an array that is tobe connected to the bonding pad pattern on a semiconductor chip and thatextend outwardly to form a package pin array; a bead of resilientinsulation material located upon said lead frame finger pattern toencompass the ends of the inwardly extending fingers; upper and lowerplates that are flat, substantially rigid and larger in area than thearea enclosed by said bead, said plates being pressed against said beadso that said outwardly extending finger portions extend outward beyondthe edges of said plates; a molded insulative ring formed around theedges of said plates whereby said plates are joined together, said ringhaving an outer dimension that determines the periphery of said packagewith said outwardly extending fingers protruding through and beyond saidmolded ring; and a semiconductor chip having its bonding pads connectedto said leadframe finger portion.
 2. The package of claim 1 wherein saidsemiconductor chip bonding pads are connected to said lead frame fingersby means of wire bonds.
 3. The package of claim 1 wherein saidsemiconductor chip bonding pads are connected to said leadframe fingersby means of assembly tape.
 4. The package of claim 1 wherein a pluralityof semiconductor chips are included.
 5. The package of claim 1 whereinsaid molded insulative ring has upper and lower faces that are flushwith the outer faces of said upper and lower plates.
 6. The package ofclaim 1 wherein at least one of said plates is metal and saidsemiconductor chip is bonded in thermal relationship to said metal. 7.The package of claim 6 wherein said semiconductor chip is soldered tosaid metal plate.
 8. The package of claim 6 wherein said semiconductorchip is cemented to said metal plate using a thermally conductingcement.
 9. The package of claim 5 wherein said plates have edges thatinclude means for keying them to said molded insulative ring whereby theplates are rigidly held in position after assembly.
 10. The package ofclaim 5 wherein said plates are composed of ceramic.
 11. The package ofclaim 10 wherein said ceramic is composed essentially of alumina andsaid semiconductor chip is secured to the inner face of one of saidplates.